The present invention relates to electrostatic chucks for holding substrates in process chamber. The present invention is also related to
In semiconductor fabrication processes, chucks are used to hold semiconductor substrates to prevent movement or misalignment of the substrate during processing. Electrostatic chucks, which use electrostatic attraction forces to hold a substrate, have several advantages over mechanical and vacuum chucks. Electrostatic chucks reduce stress-related cracks which are often caused by mechanical clamps; allow utilization of a larger portion of the substrate surface; reduce formation of corrosion particles which deposit on the substrate; and allow use of the chuck in vacuum processes.
A typical electrostatic chuck includes an electrostatic member supported by a thermally conductive base in a process chamber. The electrostatic member includes at least one electrode covered by, or embedded in, an electrical insulator or dielectric material. The electrode of the electrostatic member is electrically biased with respect to the substrate by an electrical voltage, causing electrostatic charge to accumulate in the electrostatic member. In monopolar electrostatic chucks, a plasma in the process chamber provides electrically charged species that accumulate in the substrate. The accumulated charge in the substrate has opposing polarity to the charge accumulated in the electrostatic member, resulting in an attractive electrostatic force that holds the substrate to the chuck. In bipolar electrostatic chucks, a plurality of electrodes are charged to different electrical potentials to electrostatically hold the substrate to the chuck.
During processing, the substrate and underlying chuck are exposed to high temperatures that can cause undesirable temperature gradients across the surface of the substrate. In conventional chucks, the electrostatic member is typically directly bonded to the thermally conductive base, and the heat from the substrate travels through the electrostatic member to the base. Differences in heat conduction or heat generation in conventional chucks can cause temperature gradients across the processing surface of the substrate. For example, temperatures at the perimeter of the substrate are often higher or lower than temperatures at the center of the substrate. In plasma processes, this temperature variation can occur due to differences in plasma density at the center and perimeter of the substrate. During processing, discharge of thermal energy from the plasma to the substrate increases its temperature. Higher plasma density at portions of the substrate result in higher thermal discharge and higher substrate temperatures.
The temperature variations across the substrate can also be caused by differences in electric field flux and RF energy arising from different electrical impedances across the substrate. For example, RF energy fields can accumulate at low impedance areas around the perimeter of the substrate, increasing the temperature of the substrate at its perimeter relative to its center. The higher temperatures at the perimeter of the substrate also occur when the substrate perimeter forms an overhang that is not in direct thermal contact with the thermally conductive base of the chuck. As a result, heat from the perimeter of the substrate dissipates at a lower rate than the heat from the central of the substrate. As a result, the steady state temperatures at the perimeter and center of the substrate can vary by as much as 5 to 10.degree. C., during processing of the substrate.
While temperature gradients across the substrate surface are generally undesirable, in certain fabrication processes, it may become desirable to maintain a predefined temperature profile across the substrate to compensate for other variations in processing conditions across the substrate surface, for example, process gas flow or RF plasma energy. However, uncontrolled or fluctuating temperature gradients across the processing surface of the substrate are not desirable. In deposition processes, such fluctuating temperature variations can result in deposition of a film having varying grain size, surface roughness, or film structure across the substrate, which changes the electrical properties of the deposited films. In etching processes, fluctuating temperature gradients can cause profiles of etched features to vary across the substrate surface, with some portions of the substrate having etched features that are substantially straight walled and other portions having etched features with tapered sidewalls. Also, lower temperatures can cause excessive deposition of passivating polymeric deposits (because the sticking coefficient of the polymeric deposits is higher) resulting in more tapered etched features at portions of the substrate. This variation in etched feature profile is undesirable and such excess passivating deposits are difficult to remove from the substrate.
Thus there is a need for an electrostatic chuck capable of maintaining predefined temperature profiles across the processing surface of the substrate. In certain processing applications, it is desirable to maintain the center of the substrate at higher or lower temperatures relative to the perimeter of the substrate to compensate for other variations in processing conditions across the substrate surface, for example, differences in plasma density or electrical impedances. In other processes, there is a need for an electrostatic chuck that can provide a substantially uniform and consistent temperature profile from the center to the perimeter of the substrate.